Some mobile cellular telephones, such as TDMA and CDMA satellite telephones, battery-powered fixed unmanned telephones in remote rural areas and other communications devices operate in a digital mode. Such communications comprise, typically, long periods of time idle interspersed with shorter calling periods consisting of receiving periods and transmitting periods. The transmitting periods use either constant data rates or variable data rates, combined with variable power control to account for variable amounts of interference, a distance from repeaters or base stations, and other technical factors such as fading and blocking. The interference has two components: (a) external interference and (b) self-interference from the user's own system caused by other user terminals. The instantaneous power demand can vary as a function of distance from a satellite, a repeater, or from a base station to which the terminal is transmitting. The power demand can also fluctuate due to fading and signal blockages caused by obstructions and the general RF propagation environment of the terminal.
During a normal call it can be shown that approximately 50% of the time no transmission is made from the terminal and, in the case of a variable rate vocoder, that less than 10% of the time the terminal transmits at a peak rate (e.g., 9600 bits per second) during typical 20 millisecond frames. The remainder of the time the terminal operates at half the peak rate (e.g., 4800 bps) or lower.
The terminal transmits this variable rate, which requires more power, at times when the fading and interference is also varying. As a result, a power factor for the instantaneous data rate may be 15 dB for a system operating at a peak rate of 9600 bps, to a minimum rate of 300 bps. Even greater peak power factors may exist under some circumstances. In addition, instantaneous fades of 10 to 20 dB are not uncommon, while the interference may add an additional 10 dB. The sum of these powers indicates that the frame to frame variation in required power may range up to approximately 50 dB.
The final RF transmitter power amplifier consumes a considerable amount of battery current, which increases with the data transmission rate and when RF signal impairments, such as fast fades, are present. As such, the current drawn from terminal's battery is not constant, but will typically vary considerably during operation on a frame to frame basis. Furthermore, the typical battery does not readily provide transient power to a load as is typically required for transmitters that are required to periodically transmit at a high power level. This type of operation tends to shorten battery life and lower the available battery capacity (e.g., an 1100 A-Hr battery may yield only 800 A-Hr of capacity).
Advances in materials science have resulted in the availability of so called High Energy Density Capacitors (HEDCs). One such device is manufactured by PolyStor Corporation, and is based on a very high surface area carbon aerogel material. One device, referred to as a PolyStor A-14500, has a working voltage of 2.5 V, a capacitance at 1 A of 7 Farads, an ESR at 1 kHz of &lt;35 milliohms, a specific power of 4,000 W/kg, a maximum discharge of 35 A, and a current life cycle in excess of 10.sup.5. Suggested applications for such HEDCs include hybrid power packs for digital (pulse) telecommunications applications, power conversion applications, pulse power applications such as vehicular air bag deployment, audio and electronic filtering, and as a `turbocharger` for batteries.
In general, the use of a battery is best for slow charge and non-bursting type of transmissions, while the use of the HEDC is best for the burst power case with wide dynamic range. It is an object of this invention to provide methods and apparatus for combining and exploiting the advantages of both batteries and the HEDC-type of power source.